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Nothing leaves your machine

This runs entirely in your browser. The file is read locally to identify it and draw the preview — it is not uploaded anywhere, and nothing is stored.

What it checks

Every check reads the contents of the file, never its name. Renaming a Blockbench project to .blockymodel does not make it one, and that mistake has already cost this project a finished rod — so it is the first thing this catches.

KindAcceptedWhat gets checked
Model .blockymodel Parsed as a node tree, then rendered. Box and quad shapes, node positions and quaternion orientations are all applied, so what you see is the assembled model rather than a parts pile. Hidden shapes and unrecognised shape types are reported.
Texture / icon .png Dimensions, bit depth and colour type. Warns on non power-of-two sizes, anything over 512 px, and non-square images. JPEG is rejected outright — no alpha, and its compression smears pixel art.
Sound .ogg Vorbis stream, channel count and sample rate, with playback so you can hear it. Warns on stereo, because a positional effect in stereo will not pan correctly.

A Blockbench .bbmodel is recognised and still previewed, but it is reported as an error rather than a pass: it is a project file, not something the engine can load. Export .blockymodel and check that instead. Wavefront .obj and zip archives are rejected for the same reason.